Posted by arlene

If you are using Windows 2000 Advanced Server or Windows Server 2003 Enterprise or Datacenter servers, you can use the clustering feature for DHCP. This allows two separate DHCP servers to be administered as a single DHCP server. Windows 2000/2003 clustering supports a failover mode in which a service running on one computer can be monitored. If the node that is supplying a DHCP service supported by the clustering software fails, another node that runs the same service can be activated to take over for the failed node. By clustering DHCP services between two nodes, you will make the network less prone to downtime due to problems with your DHCP server.

The alternative to clustering is to use two separate DHCP servers, each responsible for a portion of the address scope. This allows all your clients to get an address from one or the other server. Because leases are usually measured in days or weeks on a stable network, the loss of a single DHCP server for a few hours or a day or so might not cause you any problems unless someone decides to reboot every PC on the network. A secondary server configured with a smaller portion of the address space can continue to handle DHCP traffic while the main server is repaired.

In a larger network, however, where computers are frequently moved, a more stable DHCP service can be provided by hosting the DHCP service on a cluster.

Keep the following points in mind when using a Windows cluster for the DHCP service:

• The DHCP service should be installed before the clustering service is installed on the computers.
• As with most cluster installs, one server should be configured first, with the DHCP service and the clustering service, while the other cluster member-to-be is powered off.
• When you’re finished with the installation on the first member, power up the second and install the DHCP service and then the cluster service.

In addition, keep in mind that the cluster itself must have a unique IP address, which can’t be delegated to it by a DHCP server. Additionally, you’ll need to create a domain security group and make both servers members. To this group, assign Full Control permissions for the DNS zone object in the Active Directory where DHCP A and PTR records are stored for the servers‘ clients.

Using Windows clusters is the subject of many books. Before you decide to use a cluster on your network, I would recommend that you become intimately familiar with Windows clusters. There are many aspects of clustering (such as the utilities used to start/stop and otherwise manage the cluster) that you need to learn before you try to set DHCP and the clustering software.

Considerations for Using DHCP in Large or Routed Environments

In a large network you need to provide for redundancy for DHCP servers. Because a larger network is typically connected using routes to join a diverse set of network segments, you will need to enable BOOTP and DHCP forwarding on any routers in the network. Each DHCP server will need to be carefully planned, and the address scopes, reservations, and exclusions will need to be carefully thought through in advance. You don’t want, for example, a DHCP server to allocate an address to a client when that address should have been reserved and already is in use by another server! This is exactly the kind of thing automatic dispensing of IP addresses is supposed to solve.

Of course, when you’re planning the placement of DHCP servers in a large, routed environment, it’s easiest to place a single DHCP server on each subnet. In many cases, though, this is not practical. And with the forwarding capabilities it is not necessarily needed. Also, don’t yield to the temptation of placing all your DHCP servers on the same subnet, allowing them to receive forwarded replies from other network segments. If the single subnet becomes unavailable, all your DHCP servers become unavailable. This applies to any major server. Don’t place all your eggs in one basket, so to speak.

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